Well, we don't know whatĪ-sub-4 is just yet, so let's try to figure that out. Let's see what we can make of this, so a-sub-5, a-sub-5 is going to be equal to, we'll use this second line right here, a-sub-5 is going to be equal to a-sub-4 minus 2. Recursive definition of our arithmetic sequence. To be a-sub-i minus 1 minus 2, so this is actually a It in terms of the previous terms, so a-sub-i is going The arithmetic sequence a-sub-i is defined by the formula a-sub-1, they gave us the first term, and they say, every other term, so a-sub-i, they're defining This arithmetic sequence is going to be 61.
#ARITHMETIC SEQUENCE KHAN ACADEMY PLUS#
This is 57, and 4 plus 57 is equal to 61, so the 20th term in 3 times 19, let's see, 3 times 19 is 57, right? It's 30 plus 27, yep. This is going to be equal to 4 plus 3 times 20 minus 1 is 19.
Wherever we saw an i, we put a 20, and now we can just compute We would put a 20 in, so it's going to be 4 plus 3 times 20 minus 1, so once again, just to be clear, a-sub-20, where instead of a-sub-i, A-sub-20, we just use thisĭefinition of the ith term.
What is a-sub-20? And so a-sub-20 is theĢ0th term in the sequence and I encourage you to pause the video and figure out what is the 20th term? Well, we can just thinkĪbout it like this. The ith term in the sequence is going to be 4 plus 3 times i minus 1. All right, we're told that theĪrithmetic sequence a-sub-i is defined by the formula where
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